websockets: preliminary SSL support on the listening/accepting side

websockets: preliminary SSL support on the listening/accepting side

Enabled basic SSL support in the low-level websocket wrapper. Added
transport options for configuring the necessary SSL certificates
and keys. The current support is already enough to enable SSL on
the listening/accepting side (tested with the web console), although
there are a few things still missing (eg. mapping out the server side
REQUIRE_VALID_OPENSSL_CLIENT_CERT option). The client side SSL support
has not been tested.

One thing that probably needs to be changed is the websocket transport
naming scheme. It would be a better idea to follow the established 'ws:'
(plain) and 'wss:' (encrypted) prefixes.